In recent years, a light-emitting diode (hereinafter referred to as “LED”) is increasingly used as a light source of backlight for liquid-crystal display (LCD) monitors.
There have been known various systems of a power supply circuit for supplying a drive current to a load including LED. In terms of the form of output currents, LED-driving power supply circuits can be roughly classified into a circuit for supplying a constant DC current, a circuit for supplying an AC current and a circuit for supplying a pulsed current.
A power supply circuit applying the operation principle of a Royer oscillation circuit and having a push-pull circuit (hereinafter referred to as “Royer-type power supply circuit”) is known as one of means for obtaining an AC or pulsed current. The Royer oscillation circuit is disclosed in Japanese Patent Publication No. 32-4066. This Royer-type power supply circuit has two advantages: one capability of obtaining a simple circuitry, and the other capability of utilizing resonance to obtain an AC voltage analogous to sine wave. In view of these advantages, a Royer-type power supply circuit is widely used as self-oscillated inverters.
However, there are few case that a function of maintain an output current at a constant value or a function of changing the value of an output current is added to a Royer-type power supply circuit itself, for the following reason. In case of utilizing resonance, it is requited to set a resonance frequency and a switching frequency at the same value, which causes difficulties in accurately adjusting an output current as in a PWM control for changing an on-duty of a transistor.
Thus, in order to stabilize a current to be supplied from a Royer-type power supply circuit to a load, a converter circuit having a function of controlling a DC output voltage has been provided on the input side of the Royer-type power supply circuit, as shown in FIG. 1. This converter circuit is operable to control a voltage to be supplied to the Royer-type power supply circuit so as to indirectly control an AC or pulsed current to be supplied from the Royer-type power supply circuit to a load.
The above circuit will be briefly described with reference to FIG. 1. In FIG. 1, the reference numeral 1 indicates an input terminal for receiving a power from an external power source, such as a battery. The reference numerals 2a and 2b indicate output terminals for supplying a current to a load 6 with a plurality of LEDs which is connected therebetween. First and second power converter circuits 3, 10 are connected in series between the input terminal 1 and one of the output terminals 2a. The first power converter circuit 3 includes a choke coil L1, a switching transistor Q1, a rectifier diode D1, and a smoothing capacitor C1, which are connected with each other to form a step-up type converter. The second power converter circuit 10 includes transistors Q11, Q12, a capacitor C21, resistors R21, R22, and a transformer T, which are connected with each other to form a Royer-type power supply circuit.
A detector circuit 5 is connected between the other output terminal 2b and the ground serving as a reference voltage point, to detect a current flowing through a load (hereinafter referred to as “load current”) so as to generate a feedback signal in proportion to the load current. A control circuit 4 is connected between the first power converter circuit 3 and the detector circuit 5, to drive the first power converter circuit 3 in response to the feedback signal received from the detector circuit 5.
The combination of the first power converter circuit 3, the control circuit 4, the second power converter circuit 10 and the detector circuit 5 makes up a switching constant-current power supply system for supply a given current to the load 6.
The operation of the switching constant-current power supply system in FIG. 1 will be briefly described below. The control circuit 4 is operable to control the on/off action of the switching transistor Q1 with an on-duty in proportion to the feedback signal from the detector circuit 5. In conjunction with the on-off action of the switching transistor Q1, a current flows in the smoothing capacitor C1 through the rectifier diode D1, and a DC voltage appears between the terminals of the smoothing capacitor C1. The DC voltage appearing between the terminals of the smoothing capacitor C1 becomes an output voltage of the first power converter circuit 3.
In response to the output voltage supplied from the first power converter circuit 3, the second power converter circuit 10 self-oscillates to generate an alternative voltage in the secondary winding thereof. The alternative voltage generated in the secondary winding allows a pulsed current having a half-wave rectified waveform to flow in the load 6.
The feedback signal generated at the detector circuit 5 has a value in proportion to the current flowing through the load 6. Thus, the switching transistor Q1 is turned on/off with an on-duty in proportion to the value of the load current to allow the voltage between the terminals of the smoothing capacitor C1 to have a value in proportion to that of the load current. For example, if the load current is lower than a target value for stabilization, the voltage between the terminals of the smoothing capacitor C1 will be increased. In this case, the value of a current to be supplied from the second power converter circuit 10 to the load 6 is determined by the DC voltage to be supplied to the second power converter circuit 10 or the voltage between the terminals of the smoothing capacitor C1. Thus, the load current, which is lower than the stabilization target value, is led to have an increased value in response to increase in the voltage between the terminals of the smoothing capacitor C1. According to the above control process, the circuit in FIG. 1 can stabilize the load current flowing through the load 6.
In view of a stable self-oscillation in the second converter circuit 10 having a push-pull circuit, it is desired to allow both forward/backward currents to flow through the secondary winding N2. However, LED incorporated in the load 6 permits a current to flow therethrough in the only one direction. Thus, in case where a Royer-type power supply circuit is used to supply a current to a load 6 including LED, it is necessary to take some measure such that as a dummy circuit is provided in parallel to the load 6 to flow a current in a direction opposite to the forward direction of the LED, or a load 6 is designed to have a plurality of LED arrays having opposite forward directions, as shown in FIG. 2.
The descriptions of the dummy circuit and the retrodirective LED arrays are omitted because the important point in the operation of the circuit in FIG. 1 is the circuit section for implementing the current detection. The following description on another circuit will be made in the same way.
In a part of display devices/lighting devices using LED as a light source, the LED is repeatedly turned on/off at a visibly incognizable speed to adjust the brightness of a display screen (hereinafter referred to as “dimming”). Such a display device/lighting device inevitably has a period where a current flows through LED (hereinafter referred to as “current supply period”) and another period where no current flows through LED (hereinafter referred to as “current cutoff period”). Thus, in a switching constant-current power supply system where a power supply circuit for supplying a current to LED is constructed as shown in FIG. 1, the feedback signal to be supplied from the detector circuit 5 to the control circuit 4 becomes approximately zero in a current cutoff period caused by the on/off switching of the load.
In the power supply system illustrated in FIG. 1, a current flowing through the load 6 has a pulse waveform which causes a period where no current flows between pulses (hereinafter referred to as “pulse interval period”). On the other hand, the switching transistor Q1 has approximately the same switching frequency (several hundred kHz) as those of the transistors Q11, Q12. Under these conditions, even if the value of the feedback signal becomes zero in a pulse interval period, the relationship with a response speed of the feedback control system allows the control circuit 4 and the first power converter circuit 3 to be operated in the same manner as that in the period where the value of the feedback signal is not zero.
In contrast, the frequency of the on/off switching (hereinafter refereed to as “intermittence”) of LED is incredibly low as compared to the switching frequency of the power converter circuit. The actual intermittent frequency is about several hundred Hz. Thus, in terms of the phenomenon that the feedback signal becomes zero, the current cutoff period is in a different situation from the pulse interval period. Specifically, the control circuit 4 acts to maximize the on-duty of the switching transistor Q1 in the current cutoff period, and then reduce the on-duty of the switching transistor Q1 in the current supply period. In this process, the maximized on-duty in the current cutoff period causes undesirable sharp increase in the voltage between the terminals of the smoothing capacitor C1, and then a load current higher than the stabilization target value undesirably flows in the current supply period which is relatively long.
For example, as one of measures for such an unstable load current, it is contemplated to subject the feedback signal to smoothing using a capacitor having a relatively large capacity and then supply the smoothed signal to the control circuit 4. However, if a high-capacity capacitor capable of maintaining the feedback signal at a significant value is provided, the feedback signal under a processing of the control circuit 4 will be kept at a value approximately equal to an average value for a relatively long time of period. Thus, if the load current is changed due to a different factor from the intermittence, the load current deviated from the stabilization target value cannot be quickly returned to the target value. As a result, due to a different factor from the current cutoff period, the load current will be undesirably destabilized.
As above, under the condition that a load is repeatedly intermitted, a response speed of the control operation in the feedback loop of the control circuit 4-the switching transistor Q1-the smoothing capacitor C1-the second power converter circuit 10-the load 6-the detector circuit 5-the control circuit 4 cannot be likely to keep up with change in the load, resulting in out of control in the stabilization of the load current.